Connecticut Attorney General Richard Blumenthal filed suit to block the $3.8 billion acquisition of Northeast Utilities by Consolidated Edison Inc., a transaction that would create the nation's largest distributor of electricity.

In his filing with the Superior Court of Connecticut in New Britain, Mr. Blumenthal said Con Ed's high rates and poor environmental record make it a poor match for Connecticut residents. The attorney general is asking the court to overturn the decision of Connecticut utility regulators, who...